Propeller indicators are mounted on the throttle board. They indicate the speed and direction of rotation of the propulsion shaft or shafts. They also record the number of revolutions the propulsion shaft has made. The speed of rotation is important because it is related to the ship's speed. The total number of revolutions is used to determine the total distance traveled by the ship. A typical revolution counter is shown in figure 11-23

Figure 11-24.-stroboscope tachometer.

OTHER REVOLUTION INDICATORS

Equipment speed is determined by either permanently installed mechanical or electrical tachometers or by portable tachometers. Portable tachometers are hand-held, mechanical types. They require access to the end of the rotating machinery shaft.

Another type of tachometer is the stroboscope tachometer (fig 11-24). This device allows rotating machinery to be viewed intermittently, under flashing light, so that the rotation appears to stop.

WARNING

If you use a stroboscopic tachometer, NEVER reach into the rotating machinery. Although the machinery appears to be stopped, it is still rotating.

Figure 11-25.-Salinity cell and valve assembly.

Because the light is intermittent, the eye receives a series of views rather than one continuous view. To measure the speed of a machine, find the rate of intermittent light at which the machinery appears to be stopped. Then, you either read the speed of rotation directly from the stroboscope's indicator or convert the strobe's flash rate to rpm.